1. Field of the Invention
The present invention relates to an analog processing circuit, an analog integrated circuit device, an image reading device, and an image forming apparatus.
2. Description of the Related Art
Generally, in an image reading device (e.g., a scanner) mounted on an image forming apparatus (e.g., a copying machine), a sheet having an image thereon is irradiated with a light and the light reflected from the sheet is converted into an analog image signal through a charged-coupled device (CCD) line image sensor such that the image on the sheet is read. Consequently, the analog image signal is subjected to a number of analog processes in an analog front end (AFE) that is an analog integrated circuit device. The AFE finally converts the analog image signal into digital data and outputs the digital data.
The analog processes performed on the analog image signal include voltage clamping, sampling and holding, signal amplification, offset correction, and analog-to-digital conversion. First, a base voltage of the analog image signal is clamped to a reference voltage within the supply range of the AFE (i.e., voltage clamping). Then, a differential measurement is performed between a clamped voltage and the reference voltage to obtain an image signal component of the analog image signal. The image signal component is then subjected to sampling and holding.
However, there is a possibility that the reference voltage fluctuates due to the charge fluctuation (ripples) in a CCD driving signal that is used for driving the CCD line image sensor. Consequently, irrespective of whether the analog image signal supplied for performing the differential measurement is maintained at a constant level, the result keeps varying because of the fluctuation in the reference voltage. Moreover, scanning lines of the CCD line image sensor also vary in synchronization with the charge fluctuation. That results in vertical lines or waves in the image thereby degrading the image quality.
The cause of such a problem is that the clamped voltage and the reference voltage are essentially different types of voltages. More particularly, the clamped voltage of the analog image signal is obtained by clamping the base voltage of the analog image signal to the reference voltage by using the electric potential charged in a capacitor. As a result, even if there is fluctuation in the reference voltage, the clamped voltage remains, in principle, free of ripples. Meanwhile, the reference voltage uses a signal having ripples as it is. Thus, a difference in the clamped voltage and the reference voltage obtained from the differential measurement is not able to eliminate the effect of the inherent fluctuation.
Japanese Patent Application Laid-open No. H9-106590 and Japanese Patent No. 3649569 disclose a technique in which the charge fluctuation is restricted by increasing the decoupling amount of the reference voltage.
However, in the case of a high density image, even a slight fluctuation of few milivolts in the reference voltage results in image deformation. Thus, there are practical limitations in restricting the fluctuation only by adjusting the decoupling amount. Moreover, depending on a gain during signal amplification, the image deformation becomes more prominent.